Metal carriers comprising heat resistant alloy honeycomb structures housed in similarly heat resistant alloy jackets have recently come into common use as exhaust gas purifying catalyst carriers for internal combustion engines in automobiles and the like. A honeycomb structure is commonly formed by alternately laminating a flat foil with a thickness of about 50 μm with a corrugated foil obtained by corrugating the flat foil, and may be used in the form of the alternate laminate of the flat foil and corrugated foil, or as a spiral coil of stacked bands of the flat foil and corrugated foil.
In a conventional ceramic carrier, the temperature of the catalyst is too low for activation during the initial engine start-up period, and therefore most of the harmful components of the exhaust gas (HC, NOx, CO, etc.) are released during the initial engine start-up period. In contrast, a metal carrier offers numerous advantages, such as low heat capacity compared to conventional ceramic carriers, such that the heat energy of the exhaust gas itself produces rapid heating to a temperature at which the catalyst functions, for superior exhaust gas purification during the initial engine start-up period. With the increasingly rigorous restrictions on automobile exhaust gases in the U.S., Europe and Japan in recent years, demand is rising for even more rapid activation of catalysts. For this reason, there is a need for further reduction in the heat capacity of metal carriers, and this has created a demand for foil materials which are even thinner than the conventional 50 μm thickness for metal carrier foils.
The compositions employed for foil materials are commonly Fe—Cr—Al based alloys such as Fe-20 wt % Cr-5 wt % Al, as described in Japanese Examined Patent Publication HEI No. 6-8486, for example. The alloy in this composition forms a dense Al2O3 film on the surface when exposed to a high-temperature oxidizing atmosphere, and formation of the Al2O3 film inhibits the rate of oxidation and is therefore highly advantageous from the viewpoint of oxidation resistance.
As mentioned above, the need for reduced heat capacity of catalyst carriers has led to a demand for construction of metal carrier honeycombs with foils thinner than 30 μm and with lower heat capacity. On the other hand, a smaller foil thickness results in a lower absolute retention of the Al which supports oxidation resistance in the Fe—Cr—Al based stainless steel sheet, thereby reducing the oxidation resistance of the foil. The Al content therefore preferably should exceed 6.5% in order to form a metal carrier with excellent oxidation resistance, particularly when using a foil material with a thickness of less than 30 μm.
Ordinary metal honeycomb bodies are subjected to brazing with brazing materials at all or portions of the joints between foils, and in stainless steel sheets comprising Al at 6.5% or greater, an alumina film forms on the steel sheet surface during the brazing treatment and significantly impairs the wettability of the brazing material.
When a foil material is produced in mass in an ordinary steelmaking/rolling process, an Al content of greater than 6.5% in an Fe—Cr—Al steel sheet will impair the hot rolled workability and hot rolled sheet toughness, and therefore the greater number of passes required will result in the disadvantage of increased production cost. Thus, measures for improved oxidation resistance by simple increase in Al cannot be adopted in the conventional process. A demand therefore exists for a process wherein cost is not increased even by conventional means.
In a metal catalyst carrier, the Al in the metal foil is oxidized in the high-temperature exhaust gas, forming alumina (Al2O3) and consuming the Al in the foil. The Cr is oxidized next to chromium oxides, and iron chromium oxide is formed on the foil surface so that the oxidation resistance is maintained, but depletion of the Al results in deformation of the metal foil, more loss of the oxides, breaking of the foil and loss of the function as a carrier.
In order to prevent depletion of Al in the metal foil and extend the durable life of the catalyst carrier, it is effective to increase the amount of Al in the foil. In particular, a smaller metal foil thickness requires an increase in the Al concentration of the foil to ensure an absolute amount of Al. However, special processing steps are necessary when the amount of Al in stainless steel exceeds 6 wt %, while at greater than 8.0 wt % the workability is notably impaired, making rolling of the foil difficult. Especially in the case of stainless steel foils with a sheet thickness of 60 μm or smaller, materials with Al contents exceeding 7.0 wt % are poorly suited for mass production in terms of workability, and even when foil rolling is possible, corrugation results in numerous cracks and hence it is difficult to form honeycomb bodies.
In Japanese Examined Patent Publication HEI No. 4-51225 there is disclosed a process for fabrication of an exhaust gas purification catalyst wherein the surface of a stainless steel sheet with an Al content of no greater than 6.0% is plated with Al and subjected to foil rolling, and the foil is used to form a honeycomb body, after which heat treatment is carried out in a non-oxidizing atmosphere. Cold rolling and honeycomb working can be accomplished since the Al content is no greater than 6.0% at the steel sheet stage, and the subsequent heat treatment in the non-oxidizing atmosphere actively dissolves the plated Al into the steel sheet, thereby ensuring an amount of Al necessary for oxidation resistance.
For production of a metal carrier, a stainless steel flat foil and corrugated foil are alternately wound or laminated to produce a honeycomb body form, and then the points of contact between the flat and corrugated foils are brazed for bonding. For this purpose, a brazing metal is coated onto the stainless steel foil surface either after the honeycomb body has been formed or before it is formed, and the honeycomb body is heated at high temperature to melt the brazing metal and create brazed joints at the foil contact sections.
When a honeycomb body is formed using a metal foil comprising a stainless steel foil coated with Al on the surface, as described in Japanese Examined Patent Publication HEI No. 4-51225, the Al on the foil surface undergoes vaporization loss after the honeycomb body has been formed, specifically during the high-temperature heat treatment for diffusion of the foil surface Al into the stainless steel or during the high-temperature heat treatment for brazing, and in some cases the Al content of the stainless steel cannot be adequately increased. Also, the Al in the stainless steel foil surface and the brazing metal will sometimes react before brazing, during the temperature increase period prior to brazing, thus producing high melting point intermetallic compounds and impairing the bonding property at the brazed sections.
As explained above, a catalyst-carrying catalyst converter is situated in the exhaust gas path for the purpose of purifying exhaust gas from internal combustion engines. A carrier supporting a catalyst in the same manner may also be used in methanol-converting devices which perform water vapor conversion of hydrocarbon compounds such as methanol to produce hydrogen-rich gas, CO-removing devices which convert CO to CO2 for its removal, or H2-burning devices which burn H2 to H2O for its removal. Such catalyst carriers comprise numerous cells through which the gas passes, with the catalyst being coated on the walls of each cell, thereby allowing contact between the passing gas and the catalyst over a wide contact area.
Catalyst carriers which can be used for this purpose include ceramic catalyst carriers and metal catalyst carriers. For a metal catalyst carrier, a heat-resistant alloy-containing flat foil with a thickness of several micrometers and a corrugated foil are alternately wound or laminated to make a cylindrical metal honeycomb body, and the metal honeycomb body is inserted into a cylindrical metal jacket to make a metal carrier. A catalyst-supporting layer comprising the catalyst impregnated is formed on the metal foil surface of the cells of the honeycomb body which serve as the gas pathways of the metal carrier, to produce a catalyst carrier. The contacting portions of the flat foil and corrugated foil of the honeycomb body comprising the wound and laminated flat foil and corrugated foil are bonded by means such as brazing, to produce the honeycomb body as a firm structure.
The catalyst may be supported on the metal foil surface of the honeycomb body by a method in which the metal foil surfaces of the cells of the honeycomb body which serve as the gas pathways of the metal carrier are coated with a porous γ-alumina layer known as a wash coat layer and then a catalyst comprising a rare metal or the like is impregnated into the wash coat layer, or a method in which a wash coat layer containing the catalyst is coated onto the metal honeycomb body. The method for forming the wash coat layer on the cell surface of the metal honeycomb body may be a method in which the honeycomb body is immersed in the wash coat solution to attach the wash coat solution onto the cell surfaces of the honeycomb body, and is then dried to form a wash coat layer on the cell surfaces.
Japanese Examined Patent Publication HEI No. 8-197 describes a method of forming a honeycomb body using an Al-containing stainless steel metal foil, subsequently heat treating it in air and utilizing the Al in the steel to produce α-alumina whiskers on the stainless steel surface, and coating the needle-like crystals with γ-alumina, for the purpose of improving cohesion between the metal foil surface and wash coat layer of the metal honeycomb body. A method of heat treating Al-containing stainless steel in a CO2 atmosphere beforehand is described in Japanese Unexamined Patent Publication SHO No. 57-71898, as a method of accelerating production of the α-alumina whiskers.
For α-alumina whiskers to be produced on the metal foil surface it is necessary for the honeycomb body to be heat treated in air or in a specific atmosphere. Since the foil material is the source of Al for the α-alumina whiskers, the Al concentration of the foil material decreases due to the α-alumina whiskers. As a result, the original oxidation resistance of the foil cannot be exhibited.
For exhaust gas purification using a catalyst carrier, the catalyst reaction is accelerated, and exhaust gas purification efficiency improved, by more active substance movement between the exhaust gas passing through the honeycomb body cells and the catalyst on the cell surfaces.
Incidentally, the wash coat layer (γ-Al2O3) is formed on the foil surface of the honeycomb body first, after which a rare metal catalyst is loaded to produce the catalyst carrier. The loadability and high temperature stability of the wash coat on the metal carrier is important for maintaining and improving the catalyst purification performance, and various types of treatment are currently being combined for this purpose.
The wettability is poorer between stainless steel foil surfaces and wash coat layers of metal carriers, as compared to ceramic carriers such as cordierite, and therefore the loadability of the wash coat is insufficient, such that a surfactant or the like must be used for pretreatment.
The high temperature stability of the wash coat is important to maintain the specific surface ratio (a specific surface ratio of about 80-160 m2/g with 0.5-40 μm micropores) and increase the reaction efficiency. The γ-Al2O3 used for the wash coat undergoes a phase transition to α-Al2O3 from about 900° C. This leads to breakup of the microstructure of the micropores, thereby notably reducing the specific surface ratio. Thus, in order to increase the phase transition temperature and increase the thermal stability of the wash coat, a rare earth oxide such as CeO2 is dispersed in the wash coat.
The wash coat also importantly acts as a co-catalyst, adsorbing oxygen to supplement the catalytic action, and since CeO2 is also effective for this purpose it is often added in a large amount.
A metal foil with satisfactory wettability with the wash coat is preferably used for the metal carrier, since this will result in satisfactory loadability of the wash coat without pre-treatment using a surfactant or the like. Also, the metal foil of the metal carrier preferably has the power to improve the high temperature stability or oxygen-storing effect of the wash coat, since this will eliminate the need to disperse a rare earth oxide such as CeO2 in the wash coat. It is currently the case that heavy metals such as Cr or Ni, which are added to stainless steel foils and effectively improve the workability or corrosion resistance of the foils, have an oxygen-storing effect but also sometimes accelerate α-transition of γ-Al2O3, such that improvement is not easily achieved by addition of large amounts of Cr or Ni.
In a catalytic carrier for exhaust gas purification, the catalyst reaction is accelerated when the catalyst carrier reaches a temperature above its ignition point. Since the temperature of the catalyst carrier is low when the engine is started, the temperature of the exhaust gas passing through raises the temperature of the catalyst carrier and the catalyst reaction begins only after the temperature exceeds the ignition point. The time period from starting of the engine to initiation of the catalyst reaction is preferably minimized because it is during this period that the emitted exhaust gas is discharged without being purified by the catalyst. It is therefore important to increase the catalyst carrier temperature elevation rate during engine startup to improve the purification performance immediately after start-up.
The following methods have been disclosed for increasing the catalyst carrier temperature elevation rate during engine startup to improve the purification performance immediately after start-up.
Japanese Unexamined Patent Publication HEI No. 6-997976 describes an invention which is a tandem-type metal carrier wherein the sheet thickness of the honeycomb body at the exhaust gas upstream end is made smaller than the sheet thickness of the honeycomb body at the downstream end, thereby reducing heat conduction in the radial direction of the honeycomb body at the upstream end, and tending to form a heat spot. It is stated that the catalyst carrier temperature elevation time is effectively shortened by further reducing the foil thickness of the honeycomb body.
Japanese Unexamined Patent Publication HEI No. 6-320014 describes an invention wherein slits are formed in the sheet of the honeycomb at the engine-end.
Japanese Unexamined Patent Publication HEI No. 6-327973 describes an invention wherein a heating coil is provided around the carrier to allow an induction current to flow, in order to increase the temperature of the catalyst by induction heating.
Also, Japanese Unexamined Patent Publication HEI No. 9-192503 describes an invention wherein a flat foil with a thickness of no greater than 30 μm and a corrugated foil are used to construct 100-400 cells per square inch, and the outermost periphery of the honeycomb body is covered with an elastic retaining member with a heat-insulating property. The heat-insulating mechanism in combination with the thinner foil prevents heat loss from the outer periphery of the honeycomb body and improves the temperature elevation property.
The prior art inventions described above are aimed at increasing the temperature elevation rate of the catalyst carrier to improve the purification performance immediately after engine start-up. They therefore involve reducing the sheet thickness, adding slits to the flat sheet or adding a secondary heating mechanism or heat-insulating mechanism.
However, reducing the thickness of the foil lowers both the honeycomb body strength and the oxidation resistance. Foil thickness reduction is achieved when applying the invention described in Japanese Unexamined Patent Publication HEI No. 8-168680, but only up to a certain limit. Adding slits to the flat sheet will unavoidably reduce the strength of the flat sheet at those sections, while cost is also increased by the slit forming step. Furthermore, addition of a secondary heating or heat-insulating mechanism also increases the overall volume and raises costs.
Incidentally, the heat capacity of the honeycomb body can be lowered to improve the temperature elevation speed during engine start-up by reducing the thickness of the metal foil of the honeycomb body. It is known that reducing the foil thickness lowers the oxidation resistance, and methods have been proposed for increasing the Al concentration in the metal foil of the honeycomb body. However, apart from the oxidation resistance with smaller thickness, it is important to guard against problems such as flaking of the honeycomb body due to the high-temperature, high-pressure exhaust gas during use, or its collapse or break up under thermal stress.
In Japanese Unexamined Patent Publication HEI No. 5-27737 there is disclosed an invention wherein an inexpensive Y misch metal is added to an Fe—Cr—Al alloy to ensure oxidation resistance, and one or more metals from among Nb, Ta, Mo and W is further added to improve the high-temperature proof strength, the body being able to withstand a cold-heat durability test with exhaust gas at 900-1000° C.
Also, Japanese Unexamined Patent Publication HEI No. 6-389 discloses an invention wherein the metal carrier is composed of a honeycomb made of a stainless steel foil material having a high-temperature proof strength at 600° C. and 700° C. of 22 kgf/mm2 or greater and 11 kgf/mm2 or greater, respectively, with durability that can withstand a cold-heat durability test with exhaust gas at 900-1000° C.
Japanese Unexamined Patent Publication HEI No. 8-168679 discloses a honeycomb body wherein all of the points of contact between the flat sheet and foil sheet of the honeycomb body are bonded, which has excellent durability with an elastic modulus of no greater than 200 kg/mm2 in the radial direction.
Also, Japanese Unexamined Patent Publication HEI No. 8-168680 describes a honeycomb body have a foil thickness of between 17 μm and 40 μm, a proof strength of 350/t (kgf/mm2) or greater at 700° C., and a specified relationship between the Al and Cr content and the foil thickness t.
All of these prior art technologies are aimed at improving the high-temperature durability of the honeycomb body, and were developed for the purpose of increasing the proof strength at high temperature and reducing the partial elastic modulus in the honeycomb body. Methods which improve proof strength invariably lower the material workability, and also increase working costs for rolling and the like. Moreover, the effects of methods which locally reduce the honeycomb body elastic modulus are insufficient when the foil temperature increases above 1000° C.
Incidentally, it has been attempted in recent years to construct honeycombs with thin foils of 30 μm and smaller for increased heat capacity of catalyst carriers, since the heat capacity is too high with the conventional thickness of 50 μm. On the other hand, since a small foil thickness leads to lower absolute retention of Cr and Al which maintain the oxidation resistance, the oxidation resistance of a foil is proportional to its thickness, given the same chemical composition. Consequently, since the oxidation resistance of thin foils is reduced in most cases, and particularly with a thin foil of 30 μm or smaller, the alloy design must be such as to maximize the oxidation resistance above that of conventional foils. A thin foil of 30 μm or smaller preferably has an Al content of 6 wt % or greater.
When such a high-Al thin foil is used for mass production of a foil material in an ordinary steel-making, hot-rolling or cold-rolling process, the amount of Al which can be added to the Fe—Cr—Al based alloy is limited by problems such as the rolling property, and means intended to improve the oxidation resistance simply by increasing the amount of Al in an ordinary process tend to raise the rolling costs.
Published Japanese translation of PCT international publication for patent application HEI No. 11-514929 discloses a method wherein either the flat foil or corrugated foil of the honeycomb structure is an Fe—Cr—Al based alloy and the other is a layered structure comprising an Fe—Cr based alloy and an Al-containing layer, and diffusion treatment is carried out. In this method, however, Al is not enriched in the Fe—Cr—Al based alloy without the Al-containing layer, and it is difficult to obtain an Al concentration of 7 wt % or greater as a whole.
U.S. Pat. No. 4,602,001 discloses a method wherein the cell walls of a honeycomb structure composed of a steel foil with an Al content of no greater than 1 wt % are coated with Al powder, and heat treated. However, since this method employs alloy steel with an Al content of 1 wt % as the starting material, coating irregularities occurring during coating of the Al powder tend to promote abnormal oxidation at those portions.
It is an object of the present invention to solve the problems described above. Specifically, it is an object of the invention to provide an Fe—Cr—Al based stainless steel sheet and double layered sheet comprising Al at greater than 6.5% and having satisfactory hot rolled sheet ductility and excellent wettability for brazing metals, as well as a process for their fabrication,
an Fe—Cr—Al based stainless steel sheet which makes it possible to achieve improved cohesion between the metal surface and wash coat layer of the metal honeycomb body of an exhaust gas purification catalyst carrier, and an Fe—Cr—Al based stainless steel sheet or double layered sheet with excellence in terms of wash coat loading property, high temperature stability and oxygen storage property,
an Fe—Cr—Al based stainless steel sheet for fabrication of a catalyst carrier with an excellent temperature elevating property after engine start-up, i.e. with excellent exhaust gas purification performance, without radically reducing the sheet (foil) thickness and without the need for slit formation, a heating mechanism or a heat insulating mechanism,
an Fe—Cr—Al based stainless steel sheet wherein the foil material has excellent high-temperature durability, allowing its use under stringent conditions exceeding 1000° C.,
an exhaust gas purification catalyst carrier honeycomb body employing the aforementioned Fe—Cr—Al based stainless steel sheet and double layered sheet, and
a process for fabrication of a low heat capacity metal honeycomb body with sufficient oxidation resistance and excellent structural durability, even as a honeycomb body composed of an ultrathin foil.
The stainless steel sheet and double layered sheet of the invention both also encompass foils.